Device for pulling monocrystalline semiconductor rods



1966 HANS-FRIEDRICH QUAST ETAL 3,232,716

DEVICE FOR PULLING MONOCRYSTALLINE SEMICONDUCTOR RODS Filed Dec. 19. 1960 2 Sheets-Sheet 1 F b, 1, 1966 HANS-FRIEDRICH QUAST ETAL 3,232,716

DEVICE' FOR PULLING .MONOCRYSTALLINE SEMICONDUCTOR RODS 2 Sheets-Sheet 2 Filed Dec. 19. 1960 @WFMMMM MEEM Fig.5

1 Claim. 61. 23-273 Ourinv'ention relates to a device for producing monocrystalline semiconductor rods by a crucible-free crystalpulling technique. According to this technique, a monoc'rystalline seed is immersed into the molten top mound of a thick rod-shaped semiconductoringo-t, and is thereafter pulled out of the melt with the result of growing a thin monoc'rystalline semiconductor rod.

' During crystal pulling operation, the necessary heating energy is supplied to the molten top of the thick rod by means of aring-shaped heater Which surrounds the melt and preferably consists of an induction coil.

It has been. found that .with the known technique and devices ofthis kind, the melt at the top of the thick rod is subjected to ther mal variations or fluctuations which cause corresponding irregularities in the diameter of the thin monocrystalline rod being produced.

It is anobject of our invention to avoid such thermal irregularities and to afford producing a monocrystalline semiconductor rod of a greater degree of diametrical uniformity than heretofore obtainable.

, To this end, and in accordance with our invention, we supply, during pulling operation, the necessary heating energy to the molten topmound of the thick semiconductor rod by high-frequency inductance heating with the aid of a heater coil 'whose inner diameter is smaller than that of the thick-semiconductor rod, such inductance heating being applied about the nascent end of the thin semiconductor rod closely adjacent to the melt.

By thus pulling the thin semiconductor rod through a high-frequency coil which approaches the thin rod as well as the adjacent melting zone as closely as possible, the

molten zone can be keptso small that thermal fluctuations donot' occur. We have found that this not only improves the dimensional uniformity of the product, but also more reliably prevents the dripping away, melting off or freezing of the melt.

According to another feature of our invention, it is preferable to pull the thin semiconductor rod through a slitted metal cap, preferably consisting of silver, which is conductively connected with the inductance coil or one of the turns thereof and which has an inner shape adapted to that of the upwardly bulging, molten zone. With such a design of the inductive heating means, a particularly favorable configuration of the induction field is obtained.

The foregoing and more specific objects and features of our invention, said features being set forth with particularity in the claims annexed hereto, will be described hereinafter with reference to the embodiments illustrated by way of example on the accompanying drawings, in which:

FIGS. 1, 2, 3 and 4 illustrate four different embodiments of induction heater devices operating in accordance with the invention, and

FIG. 5 is a schematic and partly sectional view of a crystal pulling apparatus applicable for the purpose of the invention.

According to FIG. 1, a thick semiconductor rod 1 has its top portion melted. The molten zone forms a mound shaped top above the solid portion of the rod. The rod United States Patent 0 ice 1 consist of pure silicon, although the method is also applicable for the production of monocrystalline rods from germanium and other semiconductor materials. During pulling operation, a monocrystal seed is immersed into the melt 2 and is thereafter pulled out of the melt at uniform speed corresponding to the rate of recrystallization, so that a thin semiconductor rod 3 of constant diameter is produced. During pulling operation, the melt is kept heated by means of a high-frequency winding which is shown in FIG. 1 to comprise two spiral turns. The inner diameter of the heater 4 is smaller than the diameter of the'rod 1, and the heater is located at the nascent end of the monocrystalline rod 3 as close to the molten zone 2 as is feasible without touching the melt.

In the embodiment of FIG. 2, the thin semiconductor rod 3 is pulled through a metal cap consisting preferably of silver- The cap 5 is provided with a radial slit 7 and is connected with a high-frequency winding 6 which in this embodiment is constituted by a single turn. The winding 6 and the metal cap 5 thus form together the heating device Whose high-frequency field supplies to the melt 2 the necessary heating energy during crystal pulling operation. When performing the method, the semiconductor rod 1 is mounted in a protective gas atmosphere for example of nitrogen within a vessel consisting preferably of quartz. The thin semiconductor rod 3 may be pulled out of the vessel through a suitable seal. During processing, the heating device is moved in the direction of the arrow 12, Whereas the thick rod 1 may be fixedly held in the vessel. If desired, the thin semiconductor rod 3 can be provided with doping substance during processing.

Prior to beginning the above-described crystal-pulling operation, it is necessary to heat the top portion of the semiconductor rod 1 up to the melting temperature at which the high-frequency field becomes effective. The power for thus preheating the top of the semiconductor rod is preferably supplied by heat conductance and/or heat radiation.

Thus, according to the embodiment shown in FIG. 3, a cap 8 of molybdenum is first placed upon the top portion of the rod 1. The cap 8 is then heated to incandescent temperature by the high-frequency winding 10-. The heat thus produced in the cap 8 is'directly transmitted to the semiconductor rod 1 by heat conductance. Asa result, the electric conductance of the semiconductor material increases until the induction effect of the heater coil 4 suliices for completely melting the top and maintaining it in molten condition. The heater 10 may be used for the latter purpose after removing the cap 8 and immersing the crystal seed into the melt. However, the entire preheating device composed of parts #5 and 10 may be removed and the crystal pulling then performed by the means and in the manner described above with reference to FIGS. 1 and 2.

In the embodiment shown in FIG. 4, the preheating of the hyperpure semiconductor rod 1 which, when cold, possesses poor electric conductivity, is effected by heat radiation from a glowing molybdenum ring 9. The molybdenum ring '7 may be heated by passing electric current therethrough. For this purpose, the ring 9 may be slitted or may have one or more turns. During the pulling operation proper, the top of the rod 1 is heated by the high-frequency coil 4 in accordance with FIG. 1 or by a device as shown in FIG. 2. During pulling operation, the molybdenum ring is moved in the direction of the arrow 11 and is thus also employed for preheating the material located beneath the molten top 2 and to be melted at a later time.

The high-frequency heating can be effected by radio frequency in the manner known as such for example from the book by N. B. Hannay, Semiconductors, Reinhold Publishing Corporation, New York, 1959, Chapter 3.

The method according to the invention affords the production of thin monocrystalline rods of silicon, germanium and the like, having a substantially constant diameter along the entire length of the rod being produced, the rod diameter being smaller than 5 mm. and satisfactorily constant even if made as small as 1 mm.

By slicing the hyper-pure, thin semiconductor rod of monocrystalline constitution into Wafers of suitable thickness, and alloying or diffusing a doping substance into these wafers, various semiconductor devices such as diodes or transistors can be produced.

A complete crystal-pulling apparatus embodying a device according to FIG. 4 will be described with reference to the embodiment exemplified in FIG. 5. The apparatus comprises a base plate 13 and a top plate 14 which, together with a quartz cylinder 15, form a processing chamber to be evacuated through an exhaust duct 16. The thick semiconductor rod 1 is secured to the base 13 by means of a rigid holder 17. The seed crystal and the resulting semiconductor rod 3 are attached to a holder 18 mounted on a support 19. The support 19 is in threaded engagement with a screw spindle 20 which can be driven from the outside by an electric motor 26 and a worm gear 21 in order to lift the crystal seed and the rod 3 away from the top of the thick rod 1 to perform the crystal-pulling operation. The above-described induction heater 4 and the resistance heater 9 are mounted on a support 23 which is in threaded engagement with another screw spindle 25. Spindle 25 is revolvably journal led. An electric motor 27 is provided for revolving the spindle 25 by means of a Worm gear 28. The motor 27 is connected by electric leads to terminals 37 to re ceive therefrom the energizing current required for shifting the coil 4 and heater 9 so as to initiate and subsequently maintain the melting operation described above with reference to FIG. 4. The induction coil 4 is connected to external terminals 34- 'to Which the necessary radio-frequency voltage is applied. The resistance heater 9 is connected to external terminals 39 through which the suitable directcurrent or alternating line-frequency current is supplied. The terminals of motor 26 are denoted by 40.

It will be obvious to those skilled in the art that with respect to the particular apparatus in which the method according to our invention is performed. and the heating device according to our invention is employed, a great variety 'of modifications are available, and that devices according to the invention can be given embodiments other than particularly illustrated and described herein, without departing from the essential features of our invention and within the scope of the claim annexed hereto.

We claim:

A device for pulling relatively thin monocryst-all-ine semiconductor rods out of the molten top mound of a thick semiconductor rod after immersing a crystal seed in the melt, comprising an evacuable processing vessel,

two coaxially aligned holdersvertical ly mountedwin. said vessel for holding said two rods respectively in a vertical processing axis, one of said holders being movable axially away from the other for performing 'said pulling operation, a high-frequency heating device mounted in said vessel and coaxially surrounding the region where said molten top mound is located when said rods are inv serted into said respective holders, said heating device comprising a metal cap with a central aperture through which the end of the thin rod is pulled, said metal cap being radially slitted and covering said top mound of said thick rod when in operation, and said heating device comprising a high frequency coilcoaxially and conductively joined with said cap, the aperture of said cap having a'smaller diameter than the thick rod.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES "Zone Refin'ing by Pt'ann, John Wiley and Sons, New

r York, 1958, pages and 76.

NORMAN YUDKOFF, Primary Examiner.

GEORGE 113. MITCHELL, IVIAURICE A, BRI ND-I S'I,

Examiners, 

